1. Field of Endeavor
The present invention relates to the field of turbine technology. It relates, more specifically, to a method for machining grooves, especially fir-tree grooves, which are provided for accommodating blades, on a rotor of a, and also to a device useful for carrying out the method.
2. Brief Description of the Related Art
The fastening of the end blades, which are used on the rotor or rotating component of a low-pressure steam turbine, with fir-tree turbine blade roots, is highly stressed. In FIG. 1, an example of the rotor 10 of a low-pressure steam turbine with an exemplary rotor blade 19 is reproduced. The rotor blade 19 is inserted into one of the curved fir-tree grooves 13 in the rotor disk 11 which extend in the axial direction (axis 12). The curved fir-tree grooves 13, with the teeth 14 which are arranged between these, are reproduced in an enlarged view in FIG. 2. On their flanks, the fir-tree grooves 13 have serrations (15 in FIGS. 3 and 4) which are arranged one above the other, between which lies a serration base in each case (16 in FIGS. 3 and 4). In printed publication DE-A1-10 2007 055 842, as well as in WO-A1-01/96055, for example, a method for producing such fir-tree grooves is disclosed.
Regardless of the turbine running mode, cracks can occur in the fir-tree grooves 13. The crack damage is especially known in fir-tree grooves of the last and second-to-last rotating row on low-pressure turbine rotors of turbine types which are on the market and cannot be avoided in the current design of grooves. The incipient cracks increase further after the incubation phase and, after reaching a specific crack depth, lead to unstable crack growth with the highest possible functional risk for the operational reliability of the rotor. Therefore, incipient cracks in fir-tree grooves of turbine shafts are not acceptable.
The current prevention against component failure exists in cyclic component inspection. In this case, established supercritical crack depths are milled out in order to dissipate the increased stresses in the crack tips with growth potential. The milling out is carried out manually in part with considerable expenditure of time and qualitative uncertainty and carried out exclusively in curved fir-tree grooves with a milling machine of low material-removing capacity which is associated with a guide track of the groove bend. For this, a working platform or equipment platform, which encompasses and/or bridges the component, is especially used, which requires a high equipment and installation cost for carrying out the method.